1. Field of the Invention
The present invention relates to chemically durable porous mullite-based ceramic articles and a process for the manufacturing the same. The mullite-based ceramic articles are particularly suitable for use as a porous filtration devices and/or supports in the chemical processing industry.
2. Discussion of the Related Art
In the field of membrane separations, thin porous membranes deposited on porous supports are widely used for microfiltration and ultrafiltration of liquid media and gas separation. The macroporous support functions to provide mechanical strength for the thin porous membrane. Porous support materials include alumina, cordierite, mullite, silica, spinel, zirconia, other refractory oxides and various oxide mixtures, carbon, sintered metals and silicon carbide.
Several considerations and limitations are important in selecting the appropriate material for the porous support. The porous support should preferably exhibit the following characteristics: (1) a total porosity, as measured by Hg intrusion of greater than 30%; (2) a high permeability; and, (3) pores exhibiting good connectivity, a greater than sub-micron average pore size and a narrow size distribution. The combined effect of these properties is that the porous support will exhibit both good filtration efficiency and permeability such that the porous support will be suitable for most microfiltration and ultrafiltration applications. Lastly, for chemical processing applications, the porous support should exhibit a sufficiently high mechanical strength (MOR) and very high resistance to chemical attack. It is this last characteristic, resistance to chemical attack, that makes mullite particularly preferred for use in these chemical processing applications.
It is known to those skilled in the art, that one conventional method for making a sintered mullite structure involves firing, at about 1600.degree. C., a mixed powder of alumina (Al.sub.2 O.sub.3) and silica (SiO.sub.2), the constituent components of mullite; i.e. the reaction sintered formation of mullite bodies. Although mullite structures produced in this manner exhibit sufficient chemical resistance and mechanical strength, the mullite structures formed in this conventional manner are dense and exhibit pores of a submicron average pore size.
Mullite formation methods involving the use of pre-reacted mullite powder represent an improvement over the aforementioned reaction sintered methods. Two such reference which disclose the use of pre-reacted mullite powder include U.S. Pat. No. 4,935,390 (Horiuchi et al.) and German Pat. No. 42 26 276 (Levkov).
The Horiuchi reference discloses a method for forming a sintered mullite-based body, having improved flexural strength, involving heat treating a composition of 80 to 99.1%, and 0.1 to 20%, by weight, of a mullite powder and a sintering aid, yttrium oxide, respectively. The Levkov reference discloses a method for the production of a ceramic sintered filter body characterized in that the starting mixture consists of 90-93% mullite, having grains of between 0.63 to 0.1 mm, an opening material, either 4-8% cork scrap or 12-16% rubber scrap, having a grain size of up to a maximum of 0.2 mm, and a binder comprising 5-7% clay and 1-3% Al.sub.2 O.sub.3 ; all in weight percent. Although both of these processes result in the production of mullite bodies exhibiting larger pore sizes and improved filtration when compared to reaction sintered mullite bodies, the filtration efficiency of these mullite bodies and their chemical durability are not such that they would be suitable for use in those applications where the porous supports would be exposed to highly basic or highly acidic environments akin to those environments seen in the chemical processing industry.
A recent innovation disclosed in copending, co-assigned application, U.S. Ser. No. 60/102,621 (Brundage et al.) results in mullite bodies which are improved over those disclosed in the Horiuchi and Levkov references. The sintered mullite bodies, possessing improved filtration efficiency and permeability, are produced from a raw material composition comprised of 75 to 99% by weight pre-reacted mullite powder, and 1.0 to 25% by weight of a water-swelling clay. Although these mullite bodies exhibit improved filtration properties, the chemical durability, sufficient for most applications involving the microfiltration and ultrafiltration of liquid media and gas separation, is below that necessary for use in those chemical processing applications involving exposure to highly basic or highly acidic environments.
There is, accordingly, a clear need for a means for producing a porous mullite structure exhibiting an increased chemical durability and possessing both sufficient filtration efficiency and permeability suitable for use as porous supports in the chemical processing industry.